DTV broadcasting in the United States of America has been done in accordance with the ATSC Digital Television Standard published by the Advanced Television Systems Committee (ATSC) in September 1995 as Document A/53 and referred to simply as “A/53”. The construction of receivers for receiving DTV broadcast transmissions is described in Guide to the se of the ATSC Digital Television Standard published by ATSC in October 1995 as Document A/54 and referred to simply as “A/54”.
Customarily, the adaptive equalization filtering for a DTV receiver is digital filtering performed at baseband after the VSB AM signals are demodulated. The adaptive equalization filtering is done for suppressing multipath responses in the received signal, which multipath responses arrive via various-length transmission paths with varying amounts of attenuation. The digital filtering weights the baseband demodulation result as variously delayed and then combines the weighting results, so as to select a stronger principal one of the multipath responses that arrives via a transmission path relatively free of attenuation. The resulting equalizer response better corresponds to the modulating signal sent by the transmitter than does the baseband demodulation result supplied to the adaptive equalization filtering as its input signal. The weighting of each of the variously delayed responses that are combined to generate the equalizer response is carried forward by digital multiplication. Read-only memory can be used to implement the digital multiplications in order to achieve faster multiplication speed.
Multipath responses (or “echoes”) that precede the principal or “cursor” response in the received signal are referred to as “pre-echoes”, and multipath responses that succeed the principal response in the received signal are referred to as “post-echoes”. The use of finite-impulse-response (FIR) adaptive equalization filtering to suppress echoes generates “repeat echoes” with greater differential delay respective to the principal response. Each repeat echo arises because the original echo or a repeat echo with lesser delay is suppressed using the full spectrum of the filter input signal as differentially delayed respective to the principal response, rather than using just differentially-delayed principal response. Each repeat echo of significant energy requires a further respective digital multiplication for its suppression and gives rise to a still further repeat echo. Each repeat echo is of opposite sense of polarity from the echo that was suppressed to result in that repeat echo being generated. As long as an echo is weak, having substantially less energy than the principal component of the received signal, the repeat echo will be even more attenuated respective to the principal component of the received signal. After few cycles of repeated suppression of a weak echo and its repeat echoes, the remnant repeat echo has insignificant energy compared to the principal component of the received signal. Whether or not the energy of a remnant repeat echo has significant energy compared to the principal component of the received signal is judged on the basis of how much the results of data slicing of the response of the complete equalization filtering is affected by the remnant repeat echo.
The problem in FIR adaptive equalization filtering is that when an echo is very strong, having substantially as much energy as the principal component of the received signal, the repeat echo is not substantially attenuated respective to the principal component of the received signal. Even after many cycles of repeated suppression of a strong echo and its repeat echoes, the remnant repeat echo is likely to have significant energy compared to the principal component of the received signal. If the echo is stronger in energy than the principal component of the received signal, the repeat echoes will not be attenuated respective to the principal component of the received signal, but will successively grow respective to the principal component of the received signal.
Infinite-impulse-response (IIR) adaptive equalization filtering is preferred for suppressing post-echoes, particularly those that are substantially delayed. Such recursive filtering reduces the generation of “repeat echoes” because the original post-echo is suppressed using the response of the IIR adaptive equalization filtering, rather than using the full spectrum of the filter input signal. It is customary to cascade the IIR filtering used for suppressing substantially delayed post-echoes after FIR filtering used for suppressing pre-echoes and short-delay post-echoes, to facilitate decision-feedback equalization (DFE) being used instead of linear-feedback equalization (LFE). The preceding FIR filtering suppresses pre-echoes in the response of the subsequent IIR adaptive equalization filtering, which further reduces the generation of “repeat echoes” in the IIR filtering. Very strong echoes still present a problem because the loop gain in the IIR filtering for suppressing them becomes high enough that there is a self-oscillatory tendency in the loop.
A paper titled “A VSB DTV Receiver Designed for Indoor and Distributed Transmission Environments” was presented at the IEEE 52nd Annual Broadcast Symposium held in Washington, D.C. the ninth through eleventh of October 2002. The paper authored by R. Citta, X. Wang, Y. Wu, B. Ledoux, S. Lafleche and B. Caron described an approach to better accommodation of strong echoes that is embodied in a receiver commonly referred to as the LINX receiver, since it was designed by LINX Electronics, Inc. The LINX receiver measures the reception channel impulse response (CIR) or cepstrum. Prior to equalization the received signal is passed through a CIR-mirror filter. The CIR-mirror filter is an FIR digital filter with weighting coefficients that have values proportional to components of the CIR, but arranged with reversal in time of occurrence. The CIR-mirror filter response provides an equalizer input signal with a modified CIR that has an echo range twice that of the original CIR. All pre-echoes in the original CIR appear in the modified CIR together with post-echoes mirroring them, and all post-echoes in the original CIR appear in the modified CIR together with pre-echoes mirroring them. The modified CIR from the CIR-mirror filter is symmetrical, with the principal component of the modified CIR being centrally located. The principal component of the modified CIR from the CIR-mirror filter constructively combines the power of all the components of the original CIR. The CIR-mirror filter response generally has many more pre-echoes than the original CIR. Post-echoes delayed by many microseconds are more often encountered than pre-echoes advanced by many microseconds, unless single-frequency networks of multiple-transmitters are used in a reception area. So, the echo range of the pre-echo spectrum is usually extended in the CIR-mirror filter response. However, even though there are more pre-echoes in the modified CIR that the CIR-mirror filter provides as its response, the principal component of that response has substantially larger energy than any other component. Accordingly, repeat pre-echoes are attenuated in fewer cycles of repetition than is the case with the original CIR that has pre-echo components with substantially as much energy as its principal component.
The modified CIR having a principal component with substantially larger energy than any other component is the critical factor for successful equalization filtering, rather than that principal component combines the power of all the components of the original CIR. Combining the power of all the components of the original CIR was the rationale for using a CIR-mirror filter in the LINX receiver design. This alternative formulation of the inventive problem permits designs in which a modified CIR has an echo range substantially smaller than twice that of the original CIR. The modified CIR in these designs contains fewer echoes than the response of a CIR-mirror filter, and the sparser echo structure facilitates adaptation of the equalization filtering. These designs do not generate a pre-echo in the modified CIR for every post-echo in the original CIR, so many post-echoes can be suppressed entirely by IIR filtering.
A Ricean K-factor can be defined as the ratio of the power in the dominant path to the total power of all the echo paths, as follows:
  K  =                    ρ        0        2                              ∑                      i            =            1                    N                ⁢                  ρ          i          2                      .  In this specification a digital filter that responds to a received signal having a low K-factor with an output signal having a substantially-higher K factor is referred to as a K-factor-improvement filter or as a KFI filter, for short. The mirror filter used in the LINX receiver is a species of KFI filter. The KFI filters used in the invention are of another species, having a sparse kernel composed of a pair of non-zero weighting coefficients of equal, or substantially equal, amplitudes.